Covalent Organic Framework: An Emerging Catalyst for Renewable Ammonia Production

Abstract

Ammonia (NH3), a fertilizer feedstock and clean energy carrier, is produced primarily by the Haber‐Bosch method, which is environmentally hazardous and energy intensive. Therefore, there is an immediate need for a benign alternative, like catalytic nitrogen fixation, such as catalytic nitrogen reduction reaction (NRR). NRR must involve active, selective, scalable, and long‐lived catalysts to become a sustainable source of ammonia production. Recently, covalent organic frameworks (COFs) and COF‐based functional materials have been proposed as promising catalysts for NRR. COFs comprise repeating units of organic molecules connected by strong covalent bonds. By fine‐tuning the building blocks, various COFs can be designed with numerous active sites. This class of materials offers excellent modularity, porosity, stability, and low density. A novel type of nitrogen‐abundant COFs called covalent triazine framework (CTF) has been noted for its high electron density and ultra‐stability. This review examines the state‐of‐the‐art for the NRR using different COF‐based catalysts and discusses design principles for advancing it. The underlying mechanism of NRR with different strategies adopted for renewable ammonia production is also discussed for a clear understanding of the process. Furthermore, key parameters for improving ammonia synthesis‘s photo/electrocatalytic efficiency will also be highlighted to present new possibilities to this emerging field.

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

“…The clean and efficient photo/ electrocatalytic NRR is considered to be the solution to the high energy input and CO 2 emission of the traditional Haber− Bosch (HB) synthesis. 8 The unique properties of COFs, such as controllable porosity, backbones with sufficient active sites, narrow band gaps, and conjugated stacks, make them promising photo-NRR catalysts for achieving efficient light adsorption and charge transfer. For example, He et al reported a series of porphyrin-based Au SACs (COFX-Au; X = 1−5) by introducing electron-withdrawing and electron-donating groups at the proximal position as well as the distal position to tune the metal microenvironment (Figure 7a).…”

“…Because this NRR competes with HER at the cathode, their FE(NH 3 ) and production rate is usually very low, <40% catalysts all used phthalocyanine macrocycles to anchor metal centers because they provide abundant metal−N−C active sites, and the electron-rich feature also facilitates N 2 adsorption and activation. 8 In addition, they used strong phenazine (Figure 3i), 81 a triazine unit, 161 and dioxin 162 linkages, respectively, to enhance the COF stability in acid reaction conditions. The upgrading of catalysts may require a better theoretical understanding of reaction pathways with unique electronic structures to tune the N 2 adsorption strength and lower the activation energy barriers.…”

“…For example, oxygen reduction reaction (ORR) is the critical step in the cathode of fuel cells and metal–air batteries. − Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) dictate the efficiency of water-splitting processes. , Meanwhile, carbon dioxide reduction reaction (CO 2 RR) can convert CO 2 to value-added chemicals, such as methanol or ethanol. In addition, ammonia (NH 3 ) production through nitrogen reduction reaction (NRR) can impact the agriculture industry. − To promote those redox reactions and further develop clean energy technologies, it is imperative to develop efficient catalysts.…”

“…Although both catalysts showed excellent performance in the electrocatalytic reduction of CO 2 to carbon monoxide (CO), COF-367-Co, with a larger pore size and surface area, performed better due to its improved CO 2 adsorption capacity and accessibility to active sites. Subsequently, others suggested COF-based SACs’ applications in a wide range of clean energy conversions, including OER/ORR, HER, CO 2 RR, NRR, etc. By adopting various design strategies, they are competitive or outperform conventional catalysts in many reactions (Figure ).…”

Combination of covalent organic frameworks (COFs) and polyoxometalates (POMs): the preparation strategy and potential application of COF–POM hybrids

“…5–10 With the development of COFs, metal-based building blocks have also been employed to construct the metal based COFs, which not only enriches the types of COFs, but also offers more properties. 11–13 As reported, COFs possess a wide range of potential applications, including gas storage and separation, 14,15 catalysis, 16,17 sensing, 18,19 advanced electronics, 20,21 and drug delivery. 22,23 They are also extremely lightweight and have a high surface area, making them attractive for use in energy storage systems such as batteries 24,25 and supercapacitors.…”